1. FIELD OF THE INVENTION
This invention relates generally to the measurement of the timing accuracy of a timing source, and more particularly to a method of and apparatus for measuring the accuracy of the internal timing source of a device having a light emitting display which is energized at a rate derived from the timing cycle of that source.
2. PRIOR ART
Many electronic systems employ alpha-numeric displays for providing an indication of the results of one or more operations of that system to an operator. For example, electronic timers and clocks may employ a numeric display for providing a visual presentation of elapsed time. Electronic calculator and computer systems employ such displays for human recognition of the results of certain arithmetic operations performed by such systems. The majority of these alpha-numeric displays are of the light emitting type in which one or more elements of the display are sequentially illuminated to create the required visually recognizable character field. It has been the practice with many of these displays to multiplex the energization of each element thereof at a frequency which is related to and derived from the basic time keeping reference of the system. Such a time keeping reference is generated by an internal timing or clock source of the system, such as quartz crystal oscillator.
It is often desirable to measure the accuracy or the error, if any, of the internal timing source of such a system. If the display of such a system is capable of providing a visual indication of elapsed time, such as in an electroncic time piece, the usual practice has been to permit the time piece to operate for a relatively long period of time and compare the elapsed time thereof with an accurate standard. Any difference found in the comparison of the two readings indicates, of course, the deviation of the internal timing source from its intended operation, thereby providing an indication of its accuracy. It can be readily appreciated that this method requires that the system which is being tested be operated for a relatively long period of time in order to determine the error which may exist with any degree of accuracy. For example, if the least significant digit of the display changes in one second intervals, and the timing source has an error of one microsecond, per second or 1.times.10.sup.-.sup.4 percent, it will be necessary to operate the system for a period of approximately 12 days before a one second difference will exist between that display and the standard.
If the system display is not capable of indicating elapsed time, or if it is desirable to determine the error of the timing source in a relatively short period of time, it has been the practice to disassemble the system under test in order to gain direct access to the output of the timing source. Since the frequency of the internal timing source is usually much higher than the rate of change of the least significant digit of the associated display, such access permits measurement of the timing source error in considerably less time. Unfortunately, such disassembly of the system under test in order to gain access to the output of its internal timing source is often not easily accomplished, particularly in miniaturized systems.